1. Field of the Invention
The present invention relates to a composite optical device. More particularly, it relates to a composite optical device comprising a semiconductor laser diode and a light modulator.
2. Description of the Prior Art
Recently, semiconductor laser diodes combined with light modulators have come to be used widely as composite optical devices formed on semiconductor substrates. However, a semiconductor laser diode combined with a light modulator for modulation at a frequency as high as 40 GHz, for example, requires a multiple quantum well (MQW) active layer in a laser portion and an MQW light absorption layer in a light modulator portion that are different in composition from each other. Therefore, a semiconductor laser diode including a light modulator for modulation at a high frequency is made by forming different layers in the laser portion and etching them into specified shapes to form a semiconductor laser diode and then embedding the layers in the light modulator portion during epitaxial growth, thereby forming the light modulator. Conventional semiconductor laser diodes including light modulators made in such a process have various problems at the junction of the semiconductor laser diode and the light modulator, as described below, and are, therefore, unable to provide sufficient performance.
In the conventional semiconductor laser diode and light modulator, as shown in FIG. 11, an n-InP cladding layer 2, an InGaAs/InGaAsP MQW active layer 3, a p-InP cladding layer 4, a p-InGaAsP guide layer 5, and a p-InP cladding layer 7, located on top of the other layers, are grown on the surface of an n-InP semiconductor substrate 1. The crystalline orientation of the surface is (001). A stripe SiO.sub.2 film 8 is formed in the [110] direction on the layer 7 and the layers are etched by using the SiO.sub.2 film 8 as a mask, thereby forming a laser mesa of the semiconductor laser diode in the [110] direction. In this description, notations n- and p- represent n type and p type conductivity semiconductor materials, respectively.
Then, using the SiO.sub.2 film 8 as a selective growth mask, an n-InP cladding layer 109, an InGaAsP/AlInAs MQW absorption layer 110, and a p-InP cladding layer 111 are grown sequentially. Those layers are etched to make a modulator mesa of the light modulator linked to an end face of the laser mesa of the semiconductor laser diode.
An Fe-doped InP embedding layer and an n-InP current blocking layer are grown successively on both sides of the laser mesa and the modulator mesa. After removing the n-InP current blocking layer from an isolation region between the laser and the light modulator, a p-InP cladding layer and a p-InGaAs contact layer are grown successively, and the p-InGaAs contact layer is removed from the isolation region. Then, electrodes are formed on the p-InGaAs contact layer of the laser and on the p-InGaAs contact layer of the light modulator, which are separated by the isolation region, and an electrode is formed on the back side of the n-InP substrate, completing the conventional semiconductor laser diode and light modulator. The isolation region electrically isolates the laser and light modulator portions at the junction of the laser mesa and the modulator mesa, where a layer having relatively high conductivity is removed from the isolation region.
However, in the conventional semiconductor laser diode and light modulator, when the n cladding layer 109, the light absorption layer 110, and the p cladding layer 111 are grown on the (001) surface of the n-InP semiconductor substrate 1, after forming the laser mesa, as shown in FIG. 11, the layers also grow on the end face of the laser mesa (the end face being a plane having a (110) orientation), as shown in FIG. 11 and FIG. 12. Consequently, the n-InP cladding layer 111 overlaps the SiO.sub.2 film 8, resulting in formation of a convex portion, and the end face of the light absorption layer 110 does not squarely face the end face of the active layer 3. This results in problems such as growth failure of the p-InGaAs contact layer or the like on the layer in the convex area and failure to remove completely the n-InP current blocking layer from the isolation region, resulting in insufficient isolation of the laser portion and the light modulator. Also, the distance between the light absorption layer 110 and the active layer 3 is increased and the end face of the light absorption layer 110 and the active layer 3 cannot oppose each other, thus resulting in lower light coupling efficiency.
The foregoing description of the prior art deals with a semiconductor laser diode and light modulator, although the aforementioned problem is not limited to semiconductor laser diodes including a light modulator but is also found commonly in a junction of a semiconductor laser diode and other optical devices, such as an optical switch and an optical divider, which are formed by growing different layers on a common semiconductor substrate.